A fuse is a device which provides an electrical connection, i.e. a short circuit. Initially, a fuse connects devices, i.e., is "on." If a certain voltage or current is applied to the fuse, the fuse will open and no longer provide a connection. The open fuse is considered to be "off." In contrast, an antifuse is a device which initially does not provide a connection and is "off." After a certain voltage or current is applied to an antifuse, the antifuse provides an electrical connection and is considered to be "on." In other words, the antifuse is programmed.
For example, field programmable gate arrays (FPGAs) and programmable read-only memories (PROMs) are semiconductor integrated circuits which use fuse and antifuse structures. Typically, a PROM comprises an array of memory cells arranged in rows and columns which can be programmed to store user data. FPGAs have a large number of logic elements, such as AND gates and OR gates, which can be selectively coupled together, i.e., programmed, by means of fuses or antifuses to perform user designed functions. An unprogrammed fuse-type gate array is programmed by selectively blowing or opening fuses within the gate array. In contrast, an unprogrammed antifuse type gate array is programmed by causing selected antifuses to become conductive.
FIG. 1 is a schematic diagram of an antifuse structure 100. The antifuse has a bottom electrode 101 and a top electrode 103. The bottom electrode 101 is commonly referred to as "metal 1" and the top electrode 103 is commonly referred to as "metal 2." The bottom and top electrodes 101, 103 comprise conductive materials such as metal, polysilicon or a doped semiconductor. Typically, the conductive material comprises aluminum (Al). In some embodiments, the bottom and top electrodes 101, 103 have their own substructure comprising layers of different materials. For example, the top electrode 103 may comprise a layer of titanium nitride (TiN), a layer of tungsten (W), a layer of aluminum (Al), and another layer of titanium nitride (TiN). The layers of TiN act as barriers to prevent diffusion of the Al into the oxide layers. An intermediate layer 105 is between the top electrode 103 and the bottom electrode 101. The intermediate layer 105 comprises a material which initially has a high resistance but can be converted into low resistance by applying a programming voltage. For example, the intermediate layer 105 can be either silicon dioxide (SiO.sub.2) or amorphous silicon (.alpha.-Si). After programming, a conductive path, called a via link, connects the bottom electrode 101 to the top electrode 103.
Types of Antifuse Structures
Two different types of antifuse structures are widely used in the industry: (1) metal-.alpha.-Si-metal (amorphous silicon or .alpha.-Si) structures and (2) polysilicon-oxide-diffusion (oxide) structures. During programming, the conductive path for each type of antifuse structure forms in a different manner. In the .alpha.-Si antifuse structure, the resistivity of the intrinsic .alpha.-Si is about 10M ohm-cm. A programming voltage is applied across the electrodes of the antifuse structure which creates an electric field. When the programming voltage exceeds the breakdown voltage of the .alpha.-Si, the high electric field causes metal ions to migrate into the .alpha.-Si region and creates a filament of polycrystalline suicide. The filament forms the conductive path between the top and bottom electrodes. For example, in practice, the .alpha.-Si layer is about 800-1,000 .ANG. thick, and the programming voltage is about 12V for about 20 milliseconds.
To program the oxide antifuse structure, a high programming voltage is applied to the electrodes to ensure oxide breakdown because the breakdown field in oxide is higher than that in .alpha.-Si. Due to the resulting high current flow, a conductive path between the top and bottom electrodes is created in the oxide layer by defects and impurity migration.
However, oxide antifuse structures have several disadvantages. First, since the oxide breakdown field is high, the oxide thickness needs to be very thin (&lt;100 .ANG.) to have a reasonable value of the programming voltage (below 15V). Second, because the programming voltage is highly dependent on the thickness of the oxide layer, the oxide thickness needs to be well controlled to have a uniform programming voltage. Third, the "on" resistance of the oxide antifuse, e.g., about 400 ohms, is greater than the "on" resistance of the .alpha.-Si antifuse, e.g., about 20 ohms.
Types of .alpha.-Si Antifuse Structures
Three (3) types of .alpha.-Si antifuse structures will now be described. FIG. 2 shows a first type of antifuse structure 200. The antifuse structure 200 has a bottom electrode or metal 1 layer 201 and a top electrode or metal 2 layer 203. An insulation layer 205, typically an inter-metal-oxide (IMO) layer, is between the bottom electrode 201 and top electrode 203. A middle metal layer 207 or metal 1.5 layer is between the top and bottom electrodes 201, 203. The IMO layer 205 also extends between the bottom electrode 201 and the middle metal layer 207. An .alpha.-Si layer 209 is between the middle metal layer 207 and the top electrode 203. In the IMO layer 205 above the .alpha.-Si layer 209 is a via. The top electrode 203 is adjacent the .alpha.-Si layer 209 in the via.
However, this first type of antifuse structure 200 has several disadvantages. First, the antifuse structure 200 is not compatible with the standard silicon process. The process to form this structure requires two (2) additional masking steps to define the middle metal layer 207 and the .alpha.-Si layer 209. Second, during programming, the electric field is not uniformly distributed in the .alpha.-Si layer 209. The electric field at the bottom corners 211 of the top electrode 203 in the via is greater than the electric field in the middle of the via. The nonuniformly distributed electric field causes variation of the programming voltage. Third, after programming, the conductive path is very narrow, e.g., about 700.ANG. in diameter, and always located at a corner 211 of the via. Fourth, to accommodate for misalignment, the size of the antifuse structure is much larger than the via.
FIG. 3 shows a second type of .alpha.-Si antifuse structure 300. The second type of antifuse structure 300 has a bottom electrode 301 or metal 1 layer, and a top electrode 303 or metal 2 layer. An IMO layer 305 is between the bottom electrode 301 and top electrode 303. An .alpha.-Si layer 307 is deposited on top of the bottom electrode 301. A conductive layer 309 is deposited on top of the .alpha.-Si layer 307. For example, the conductive layer 309 comprises TiN or TiW. The conductive layer 309 acts as an etch stopper to prevent variation of the .alpha.-Si layer 307 during the via etch step. The IMO layer 305 has a channel or via above the conductive layer 309. The top electrode 303 extends into and fills the via.
This second type of .alpha.-Si antifuse structure 300 has some advantages over the first type of .alpha.-Si antifuse structure 200. First, during programming, the electric field is more uniformly distributed than the electric field of the first type of .alpha.-Si antifuse structure. The planar structure of the top electrode 303 into the via does not have the sharp corners which cause a nonuniform electric field. Second, the process to make the second type of .alpha.-Si antifuse 300 is less complex than the process to make the first type of .alpha.-Si antifuse 200. For the second type of antifuse structure 300, one masking step is used to define the conductive layer 309 and the .alpha.-Si layer 307.
However, the process to make the second type of .alpha.-Si antifuse 300 has some disadvantages. The structure requires a special etch technique to etch the conductive layer 309, and .alpha.-Si layer 307, but not etch the bottom electrode 301. In addition, to accommodate for misalignment, the size of the antifuse structure is much larger than the via.
FIG. 4 shows a third type of .alpha.-Si antifuse structure 400 used by "QuickLogic." The third type of antifuse structure 400 has a bottom electrode 401 or metal 1 layer, a top electrode 403 or metal 2 layer, and an insulation layer 405. The insulation layer 405, i.e. IMO layer, is adjacent the bottom electrode 401. A tungsten plug 407 is in the insulation layer 405 and adjacent the bottom electrode 401. The tungsten plug 407 also functions as part of the bottom electrode 401. An .alpha.-Si layer 409 is adjacent a portion of the insulation layer 405, the tungsten plug 407 and the top electrode 403. This structure 400 is similar to an upside-down version of the first structure 200. During fabrication of the third type of antifuse structure 400, after depositing the tungsten plug 407, a chemical-mechanical-polish (CMP) process is used to ensure a flat surface before the .alpha.-Si layer 409 deposition. CMP is a planarization technique which improves global planarity, step coverage, interconnect speed and process windows. CMP is gaining wide acceptance in the fabrication of deep sub-micron technologies.
This third type of .alpha.-Si antifuse 400 also has some disadvantages. First, during programming, the electric field is not uniformly distributed at the corners of the tungsten plug 407. Second, to adjust for misalignment, the size of the .alpha.-Si layer 409 is much larger than the via size. However, the third structure 400 requires only one masking step to pattern the .alpha.-Si layer 409.
As device sizes continue to decrease while chip density increases, there is a need to decrease the size of antifuse structures. In all the above processes, the .alpha.-Si antifuse mask needs to align to the bottom electrode. The top electrode also needs to align to the .alpha.-Si antifuse mask. Therefore, the antifuse device sizes are relatively large to accommodate for the misalignment between the .alpha.-Si layer and the bottom and top electrodes. What is needed is a method and structure for a self-aligning antifuse to allow for reduction of antifuse size. The present invention discloses such methods and structures.